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Title: Performance Evaluation and Beneficiation of High Sulfur and High Alkali Off-spec Fly Ashes

Author(s): Gopakumar Kaladharan

Publication: Web Session



Appears on pages(s):



Date: 4/1/2021

Coal-fuel power plants with semi-dry or dry flue gas desulfurization (FGD) systems produce high sulfur and/or high alkali (HSA) fly ashes due to comingling of the ash with FGD products. Such fly ashes do not meet ASTM C618’s SO3 content limit or are unable to mitigate the alkali-silica reaction (ASR) in concrete. The chemistry and mineralogy of the sulfur present in HSA ash can vary significantly (e.g., CaSO4, CaSO3, Na2SO4) based on the FGD technology used and this in turn affects the performance of these fly ashes in cementitious systems. Thus, the single SO3 limit prescribed by ASTM C618 is not sufficient to capture the complexity and performance of HSA fly ashes and this results in elimination of potentially viable SCMs. The objective of this study is to evaluate the effect of HSA fly ash on various performance parameters such as workability, pore fluid pH, setting time, strength development, and potential for deleterious expansion in paste and mortar mixtures. To understand these effects, the study considers both real HSA fly ashes and simulated systems (by blending of spec fly ash with sulfur compounds). In cases where poor performance is observed, the mechanism of deleterious behavior is explored and potential beneficiation options for improving the performance are offered and evaluated.